1. Field of Invention
The present invention relates to a method of producing a container for holding a fragile substrate in a cylindrical housing, with a shock absorbent member wrapped around the substrate, for use in a fluid treatment device, and more particularly to a method of producing a catalytic converter for holding a catalyst substrate of a honeycomb structure, with a shock absorbent mat wrapped around it in a cylindrical housing.
2. Description of Related Arts
In recent automotive vehicles, a catalytic converter, a diesel particulate filter (abbreviated as DPF) and the like have been equipped. In order to produce them, generally employed is such a method for wrapping a shock absorbent member around a fragile ceramic catalyst substrate (or, filter), and stuffing them into a cylindrical housing (casing), with the shock absorbent member being compressed.
For example, Japanese Patent Laid-open Publication No. 2001-355438 proposes a method of producing a catalytic converter, by measuring the outer diameter of a catalyst substrate, when the catalyst substrate with a holding material mounted around its periphery is stuffed (pressed) into a holding cylinder, and then stuffing the catalyst substrate with the holding material mounted thereon into the holding cylinder with its inner diameter adapted for the measured outer diameter. Also, it is proposed to measure the outer diameter of the holding material mounted on the catalyst substrate, and stuff the catalyst substrate with the holding material mounted thereon into the holding cylinder with its inner diameter adapted for the measured outer diameter. Furthermore, it is proposed to measure the outer diameter of the holding material in such a state that a certain pressure is applied to the holding material. It is also proposed to select a holding cylinder having a proper inner diameter, out of a plurality of holding cylinders with various inner diameters different from one another, which were provided in advance.
In contrast, it is proposed such a method called as “sizing” or “calibrating”, wherein after the catalyst substrate and a shock absorbent mat mounted thereon were inserted into a cylindrical member, the diameter of the cylindrical member is reduced until the shock absorbent mat will be compressed to the most appropriate compressed amount, as disclosed in Japanese Patent Laid-open Publication Nos. 64-60711, 8-42333, 9-170424, 9-234377, U.S. Pat. Nos. 5,329,698, 5,755,025, 6,389,693, and European Patent Publication No. EP0982480A2 and so on. Among them, in Japanese Patent Laid-open Publication No. 9-234377, it is proposed to reduce a casing along its entire longitudinal length, in order to solve a problem in its prior art as disclosed in Japanese Patent Laid-open Publication No. 2-268834. In the former Publication, it is stated about the latter Publication that there is disclosed a catalytic converter with a central portion of a tubular body reduced in diameter to form a compressed portion, and compress a support mat to support a ceramic honeycomb body in the casing. And, it is stated in the former Publication that the above problem will be caused, as a clearance between the outer circumference of the honeycomb body and the inner circumference of the casing is large in a direction from an end of the compressed portion toward cone portions which are not reduced in diameter.
According to the conventional method by the stuffing process as described above, on the basis of density of a shock absorbent mat served as the shock absorbent member, which is called as GBD (abbreviation of gap bulk density), an annular clearance between the outer diameter of the catalyst substrate and the inner diameter of the cylindrical housing is determined, in general. The GBD is the value obtained from [weight per unit area/bulk gap]. According to the bulk density of the shock absorbent mat, pressure (Pascal) is created to hold the catalyst substrate. The pressure has to be adjusted to a value which will not exceed the strength of the catalyst substrate, and to a value which is capable of holding the catalyst substrate applied with vibration and exhaust gas pressure not to be moved in the cylindrical housing. Therefore, the shock absorbent member (shock absorbent mat) is required to be stuffed to create the GBD within a predetermined design range, and the GBD is required to be maintained for a life cycle of the product.
According to the conventional method by the stuffing process as described above, however, an error in the outer diameter of the catalyst substrate necessarily caused when producing it, an error in the inner diameter of the cylindrical housing, and an error in weight per unit area of the shock absorbent mat disposed between them are added to create an error in GBD. Therefore, it can not be a practical solution for mass-production to find a combination of each member adapted to minimize the error in GBD. Furthermore, the GBD itself is varied depending upon the property or individual difference of the shock absorbent mat. And, the GBD relies on the value measured on a flat plane, so that it does not indicate the value measured in the case where the shock absorbent mat is tightly wrapped around the catalyst substrate. Accordingly, it has been desired to stuff the catalyst substrate properly into the cylindrical housing, without relying on the GBD.
On the contrary, according to the conventional sizing method, it is proposed to measure the outer diameter of the catalyst substrate and the inner diameter of the cylindrical housing in advance, to determine an appropriate compression amount for the shock absorbent member, and then reduce the diameter by the determined compression amount. However, it is difficult to determine whether the final compression amount is appropriate or not. This is because when reducing the diameter of the metallic cylindrical member, it is required to reduce the diameter slightly smaller than a target diameter (so called overshooting), in view of a spring back of the cylindrical member. As a result, excessive compression force might be created. Also, a further difficulty is resulted from the fact that when reducing the diameter of the metallic cylindrical member, unavoidable change in thickness of its wall is caused.
In order to solve the problem caused by the overshooting or the like as described above, such a method for measuring the outer diameter of the catalyst substrate in advance, and reducing the diameter of the housing on the basis of the compression amount or target thickness of the shock absorbent mat has been proposed, in the U.S. Pat. Nos. 5,755,025, 6,389,693 and European Patent Publication No. EP0982480A2 as cited before. However, nothing is considered about the various errors caused with respect to the shock absorbent mat including the error in weight per unit area of the shock absorbent mat as described before. Therefore, the ultimate problem about the error in pressure applied to the catalyst substrate can not be avoided.
With respect to a holding force for holding the catalyst substrate in a predetermined position within the cylindrical housing, the holding force in a radial direction of the cylindrical housing corresponds to the pressure reproduction force of the shock absorbent mat acting on the outer surface of the catalyst substrate and the inner surface of the cylindrical housing, in a direction perpendicular to those surfaces. On the other hand, with respect to the cylindrical housing fixed to the exhaust system for the automotive vehicle, for example, the catalyst substrate and shock absorbent mat are applied with force in their axial directions, due to vibration or exhaust gas pressure. In opposition to the axial force, a holding force is required for them in the axial (longitudinal) direction of the cylindrical housing, which holding force is created by first frictional force between the shock absorbent mat and the catalyst substrate, and second frictional force between the shock absorbent mat and the cylindrical housing.
The first and second frictional forces are indicated by the product of multiplying the pressure reproduction force of the shock absorbent mat and the coefficient of static friction between the shock absorbent mat and the outer surface of the catalyst substrate, and the product of multiplying the pressure reproduction force of the shock absorbent mat and the coefficient of static friction between the shock absorbent mat and the inner surface of the cylindrical housing, respectively. In this respect, as for the holding force in the axial (longitudinal) direction of the cylindrical housing, the frictional force between the shock absorbent mat and the remaining one with the smaller coefficient of friction is dominant. With respect to the catalyst substrate and cylindrical housing with known coefficients of static friction, therefore, frictional forces are made clear. In order to ensure the requisite frictional forces, it is required to increase the pressure applied to the shock absorbent mat. In the case where the catalyst substrate is fragile, it is required to ensure the axial holding force within the pressure limit to the shock absorbent mat, to avoid excessive radial load applied to the catalyst substrate.
Accordingly, it is preferable to determine the pressure applied to the shock absorbent mat, on the basis of the one with the smaller coefficient of static friction, out of the coefficient of static friction of the outer surface of the catalyst substrate and the coefficient of static friction of the inner surface of the cylindrical housing, and reduce the diameter of the cylindrical housing in accordance with the determined pressure. In the prior methods, however, generally employed is a control on the basis of the GBD of shock absorbent mat as described before, so that a control through an estimation on the basis of a substituted value has been employed. Therefore, those estimated factors are added together to cause the unavoidable error. Also, the holding force that is caused by the frictional force between the shock absorbent mat and catalyst substrate, and the holding force that is caused by the frictional force between the shock absorbent mat and cylindrical housing, are eventually confused with each other, to determine the dimensions of each parts.
As a result, when holding the catalyst substrate in the cylindrical housing with the shock absorbent mat disposed between them, most appropriate parameter is the pressure (Pascal) applied to the substrate (catalyst substrate, or filter) through the shock absorbent mat (shock absorbent mat). If it is possible to measure the pressure directly, or measure a value directly corresponding to or similar to the pressure, and reduce the diameter of the cylindrical housing on the basis of one of the measured results, then it is possible to reduce the diameter of the cylindrical housing by a sizing process, with satisfactory accuracy.
However, it is very difficult to measure the above-described pressure itself directly. Especially in the case where the shock absorbent mat and catalyst substrate have been accommodated in the cylindrical housing, with the pressure created by the reaction force of the shock absorbent mat, it is required to insert a measuring device into the cylindrical housing so as to measure the pressure, and then remove the measuring device out of the cylindrical housing after measurement, which is too difficult to provide a realistic solution. Alternatively, it could be proposed to measure strain or the like created on the cylindrical housing, and use it as a factor indicative of the pressure. However, a sufficient accuracy required for the measured pressure could not be obtained.